3. MATCHING AVAILABLE FORAGE TO ANIMAL NEEDS

After you have estimated the available forage in a field using either the RPM or NDVI mapping, you will need to calculate the stocking rate for each paddock. Stocking rate is a function of the number of animals per paddock, the length of the grazing period, and the size of the paddock. Therefore, the way in which you calculate stocking rates will depend on whether the number of paddocks or their size are adjustable or not.

3.1 Steps to determine the length of the grazing period in each paddock if the current number or size of paddocks is already set

Note: The example calculations provided here are based on a fictitious 40-acre tame pasture subdivided in five paddocks sized 6, 6, 8, 10 and 10 acres, respectively. Additionally, we will consider a fictitious 30 cow-calf pair herd in which each pair weighs between 1,100 and 1,300 lbs. We are aiming to turn the herd in by June 17.

Step 1: Estimate how much forage is available in each paddock.

In this step, you will use the data obtained from either the RPM or NDVI technologies. For the sake of example, consider you obtained the following forage data from each paddock:

Paddock 1 (6 acres): 4,800 DM lbs

Paddock 2 (6 acres): 4,800 DM lbs

Paddock 3 (8 acres): 7,200 DM lbs

Paddock 4 (10 acres): 9,750 DM lbs

Paddock 5 (10 acres): 7,250 DM lbs

Step 2: Estimate how much forage can be used in each paddock.

In this step, you will consider a utilization rate based on what kind of pastures you have. Tame pastures can support higher utilization rates (up to 75%) while native pastures support a lower utilization rate (up to 50%).

Equation: Usable forage in each paddock-1 (DM lbs) = available forage in paddock DM lbs x utilization rate

For the example provided, consider a utilization rate of 50% for each paddock, as follows:

Example:

Usable forage in Paddock 1:

4,800 DM lbs x 0.5 = 2,400 DM lbs

Usable forage in Paddock 2:

4,800 DM lbs x 0.5 = 2,400 DM lbs

Usable forage in Paddock 3:

7,200 DM lbs x 0.5 =3,600 DM lbs

Usable forage in Paddock 4:

9,750 DM lbs x 0.5 = 4,875 DM lbs

Usable forage in Paddock 5:

7,250 DM lbs x 0.5 = 3,625 DM lbs

Step 3: Estimate the daily forage demand of the herd.

In this step, you can either use the forage demand of the herd based on the number of animal unit equivalent (AUE) it has (as described earlier in Figure 1), or you can calculate forage demand based on the weight of each animal unit. If you decide to calculate the forage demand based on the weight of each animal unit, you can consider a daily consumption of 3.5% (both intake and trampling) relative to the animal unit’s weight.

Equation: Demand per animal unit (DM lbs/day) = Average weight of animals (lbs) x 0.035

Once you know the daily demand of each animal unit, you can calculate the daily consumption of the herd.

Equation: Herd’s daily demand (DM lbs/day) = number of animal units x daily demand of each animal unit

Our example herd consists of 30-cow-calf pairs with weights ranging from 1,100 to 1,300 lbs each. In this case, we will assume an average weight of 1,150 lbs.

Example:

Cow-calf pair’s demand = 1,150 x

0.035 = 40.2 DM lbs/day

Herd demand DM lbs/day = 30 *

40.2 = 1,206 DM lbs/day

Step 4: Determine the length of period your herd can graze in each paddock.

In this step, you will calculate the ideal number of days your herd can graze in each paddock based on the amount of usable forage available in each paddock and on the daily demand of herd.

Equation: Number of animal days per paddock = usable forage per paddock / herd’s daily forage demand

Example:

Number of animal days in Paddock 1

2,400 / 1,206 = 2 days

Number of animal days in Paddock 2

2,400 / 1,206 = 2 days

Number of animal days in Paddock 3

3,600 / 1,206 = 3 days

Number of animal days in Paddock 4

4,875 / 1,206 = 4 days

Number of animal days in Paddock 5

3,625 / 1,206 = 3 days

By using available forage data acquired through either the RPM or NDVI, it was calculated that a herd comprised of 30 cow-calf pairs can graze in each of the five paddocks in this 40-acre field for 2, 2, 3, 4 and 3 days, respectively, totaling 14 days.

3.2 Steps to determine the number and size of paddocks needed if these are not yet set

Note: The example calculations provided here are based on a fictitious 30-acre tame pasture and a 30 cow-calf herd weighing between 1,100 and 1,300 lbs, like in the previous example. We are aiming to turn the herd into this field by July 1.

Step 1: Estimate how much forage is available per acre.

In this step, you will use the forage data obtained from either the RPM or NDVI technologies. For our 30-acre example field, we can assume we found productivity of 1,200 DM lbs/acre.

Step 2: Estimate how much forage can be utilized per acre.

Equation: Average usable forage (DM lbs/acre): Available forage (DM lbs/acre) x utilization rate

For our 30-acre example field, we can consider a utilization rate of 50%.

Example:

Average usable forage: 1,200 DM lbs x 0.5 = 600 DM lbs/acre

Step 3: Estimate the daily forage demand of the herd.

Since this is the same herd as in section 3.1, we can assume the same forage requirement of 1,206 DM lbs/day.

Step 4: Decide the duration of the grazing period.

In this step, you will decide on how long you want each paddock within a field to be grazed during each grazing event.

The duration of the grazing period in each paddock will depend on your management strategy and pasture conditions. Additionally, factors such as labour costs (e.g., time spent moving the herd) and material costs (e.g., fuel costs, fences, water troughs) should be taken into consideration when deciding on the duration of the grazing period.

In addition to the grazing period, you also need to decide on the duration of the rest period in each paddock. For Alberta conditions, a rule-of-thumb is to give rotationally grazed pastures at least 30 days of rest before they are re-grazed, but this will depend on the time of the year and pasture conditions. In general, longer rest periods are required in the early fall when growth slows down, while in the spring when growth is faster, shorter rest periods may be sufficient.

For our example we can consider a three-day grazing period.

Step 5: Calculate the size of each paddock.

In this step, you will calculate the average size of each paddock based on the average usable forage per acre, the herd’s forage demand and the duration of the grazing period.

Equation: Paddock size = Daily herd forage demand x grazing period / usable forage

Example:

Paddock size = 1,206 x 3 / 600 = 6 acres

Step 6: Calculate the number of paddocks.

In this calculation, you will simply divide the total area of the field by the calculated average acre size obtained in the previous step.

Equation: Number of paddocks = Field total size / calculated paddock size

Example: Number of paddocks = 30 / 6 = 5 paddocks

By using the available forage data acquired through the RPM or NDVI to do the calculations above, it was found that in order to support a herd of 30-cow-calf pairs for five days in each paddock, this 30-acre pasture should be divided into five paddocks of six acres each.